Method and apparatus for cooling hot-rolled steel strip

Abstract

Provided are a cooling method and a cooling apparatus that, in the cooling of a hot-rolled steel strip, regulates the amount of cooling water in a two-stage manner for each set of headers in the width direction and changes the rate at which the steel strip is cooled, in a multistage manner by a simple method, and that is effective particularly in cooling the lower surface of the steel strip, where space is narrow. The spray nozzles 5 are arranged in a row in the width direction of the steel strip at a predetermined pitch. Two systems of cooling headers 6 are arranged for one set so that spray nozzles 5 adjacent in the width direction can be supplied with cooling water from different pipe systems, and a spray valve 7 is attached to each cooling header 7 so that spraying/stop of spraying of cooling water can be individually performed.

The invention claimed is: 
     
       1. A method for cooling a hot-rolled steel strip, the method comprising:
 preparing a cooling apparatus including at least a couple of cooling headers having a plurality of spray nozzles arranged in a width direction,
 the cooling headers being arranged in a steel strip conveying direction, 
 spray nozzles of one of the couple of cooling headers are arranged on a straight line extending in the width direction, and spray nozzles of another of the couple of cooling headers are arranged on the straight line between respective spray nozzles of the one of the couple of cooling headers, 
 supply of cooling water for the one of the couple of cooling headers and for the other of the couple of cooling headers being performed using two systems of supply pipes, respectively, 
 a valve being attached to each of the two systems of supply pipes of cooling water so that spraying or stop of spraying of cooling water from the spray nozzles of the one of the couple of cooling heads and spraying or stop of spraying of cooling water from the spray nozzles of the other of the couple of cooling headers can be independently performed, 
 
 wherein when increasing cooling rate, supplying cooling water to both of the one and the other of the couple of cooling headers from both of the two systems of supply pipes and spraying cooling water from all of the spray nozzles of the couple of cooling headers, and 
 wherein when decreasing cooling rate, supplying cooling water to any one of the one and the other of the couple of cooling headers from a corresponding one of the two systems of supply pipes and spraying cooling water from every other spray nozzle of the couple of cooling headers in the width direction. 
 
     
     
       2. The method for cooling a hot-rolled steel strip according to  claim 1 , wherein
 two couples of the cooling headers in the steel strip conveying direction are referred to as a pair, 
 in each pair, the spray nozzles of one of the couple of the cooling headers are placed at the same position in the width direction as the spray nozzles of the other couple of cooling headers, and 
 when spraying cooling water from any one of the two systems of supply pipes in each pair, the spray nozzles of the one of the couple of cooling headers and the spray nozzles of the other of the couple of cooling headers spray cooling water from alternate positions in the width direction. 
 
     
     
       3. The method for cooling a hot-rolled steel strip according to  claim 1 , wherein the spray nozzles have a rectangular or elliptic spray pattern, and are arranged in such a manner that, when performing supply of cooling water from two systems and when cooling water collides with the steel strip, the position of the end of the spray colliding part collides with a position located on the opposite side of the central axis of the adjacent nozzle from the nozzle spraying cooling water and located 0 to 30 mm from the central axis of the adjacent nozzle. 
     
     
       4. The method for cooling a hot-rolled steel strip according to  claim 1 , wherein
 two couples of the cooling headers in the steel strip conveying direction are referred to as a pair, and 
 in each pair, the spray nozzles of one of the couple of the cooling headers are placed at the same position in the width direction as the spray nozzles of the other of the couple of cooling headers, and 
 the spray nozzles in a pair are displaced in the width direction by ½ of nozzle pitch from the spray nozzles in another adjacent pair. 
 
     
     
       5. The method for cooling a hot-rolled steel strip according to  claim 1 , wherein the upper surface and lower surface of the steel strip differ in cooling water amount density, and, in each cooling headers for the upper surface and lower surface of the steel strip, the number of supply pipes for cooling water is changed individually. 
     
     
       6. The method for cooling a hot-rolled steel strip according to  claim 1 , wherein the method is applied to cooling of the lower surface of the steel strip. 
     
     
       7. A cooling apparatus for cooling a hot-rolled steel strip, comprising:
 at least a couple of cooling headers having a plurality of spray nozzles arranged in a width direction, the cooling headers being arranged in a steel strip conveying direction, spray nozzles of one of the couple of cooling headers are arranged on a straight line extending in the width direction, and spray nozzles of another of the couple of cooling headers are arranged on the straight line between respective spray nozzles of the one of the couple of cooling headers, 
 wherein supply of cooling water for the one of the couple of cooling headers and for the other of the couple of cooling headers is performed using two systems of supply pipes, respectively, a spray valve is attached to each of the two systems of supply pipes of cooling water so that spraying or stop of spraying of cooling water from the spray nozzles of the one of the couple of cooling headers and spraying or stop of spraying of cooling water from the spray nozzles of the other of the couple of cooling headers can be independently performed, and 
 wherein the apparatus includes a control mechanism that makes it possible to, when increasing cooling rate, supply cooling water to both of the one and the other of the couple of cooling headers from both of the two systems of supply pipes and spray cooling water from all of the spray nozzles of the couple of cooling headers, and to, when decreasing cooling rate, supply cooling water to any one of the one and the other of the couple of cooling headers from corresponding one of the two systems of supply pipes and spray cooling water from every other spray nozzle of the couple of cooling headers in the width direction. 
 
     
     
       8. The apparatus for cooling a hot-rolled steel strip according to  claim 7 ,
 wherein two couples of the cooling headers in the steel strip conveying direction are referred to as a pair, and 
 in each pair, the spray nozzles of one of the couple of the cooling headers are placed at the same position in the width direction as the spray nozzles of the other couple of cooling headers, and 
 wherein the apparatus has a control function capable of opening and closing the spray valves in such a manner that, when spraying cooling water from any one of the two systems of supply pipes in each pair, the spray nozzles of the one of the couple of cooling headers and the spray nozzles of the other of the couple of cooling headers spray cooling water from alternate positions in the width direction. 
 
     
     
       9. The apparatus for cooling a hot-rolled steel strip according to  claim 7 , wherein the spray nozzles have a rectangular or elliptic spray pattern, and are arranged in such a manner that, when cooling water collides with the steel strip, the position of the end of the spray colliding part is located on the opposite side of the central axis of the adjacent nozzle from the nozzle spraying cooling water and is located 0 to 30 mm from the central axis of the adjacent nozzle. 
     
     
       10. The apparatus for cooling a hot-rolled steel strip according to  claim 7 , wherein
 two couples of the cooling headers in the steel strip conveying direction are referred to as a pair, and 
 in each pair, the spray nozzles of one of the two couples of the cooling headers are placed at the same position in the width direction as the spray nozzles of the other of the two couples of cooling headers, and 
 the spray nozzles in a pair are displaced in the width direction by ½ of nozzle pitch from the spray nozzles in another adjacent pair. 
 
     
     
       11. The apparatus for cooling a hot-rolled steel strip according to  claim 7 , wherein the apparatus has a control function that, when two-system cooling water is supplied, is capable of spraying in such a manner that the upper surface and lower surface of the steel strip differ in cooling water amount density, and is capable of opening and closing the spray valves in order to change the number of supply systems for cooling water individually, in each cooling headers for the upper surface and lower surface of the steel strip. 
     
     
       12. The apparatus for cooling a hot-rolled steel strip according to  claim 7 , wherein the apparatus is applied to cooling of the lower surface of the steel strip.